2 14 Darwin^ and after Darwin. 



policy in question fails. In other words, where the 

 inhabitants of an area are free to migrate to other 

 areas, the policy of correlating affinity with distribu- 

 tion is most significantly forgotten. In this case 

 species wander away from their native homeS; and 

 the course of their wanderings is marked by the 

 origination of new species springing up en route. 

 Now, is it reasonable to suppose that the mere cir- 

 cumstance of some members of a species being able 

 to leave their native home should furnish any occasion 

 for creating new and allied species upon the tracts over 

 which they travel, or the territories to which they go? 

 When the 400 existing species of humming-birds 

 have all been created on the same continent for some 

 reason supposed to be unknown, why should this 

 reason give way before the accident of any means 

 of migration being furnished to humming-birds, so 

 that they should be able to visit, say the continents 

 of Africa and Asia, there gain a footing beside the 

 sun-birds, and henceforth determine a new centre for 

 the separate creation of additional species of hum- 

 ming-birds peculiar to the Old World — as has hap- 

 pened in the case of the majority of species which, 

 unlike the humming-birds, have been at any lime 

 free to migrate from their original homes ? 



Lastly, my third consideration is, that the supposed 

 policy in question does not extend to affinities which 

 are wider than those between species and genera — 

 more rarely to families, scarcely ever to orders, and 

 never to classes. In other words, nature shows a 

 double correlation in her geographical distribution 

 of organic types: — first, that which we have already 

 considered between geographical restriction and 



